[tracker/wip/carlosg/property-function-graph-check: 1/2] libtracker-data: Check graph existence in property functions

[Carlos Garnacho <carlosg src gnome org>]

commit 4988d9f345554a28d2109c33fcfcc36afc8aac7e
Author: Carlos Garnacho <carlosg gnome org>
Date:   Thu May 28 00:58:01 2020 +0200

    libtracker-data: Check graph existence in property functions
    
    And resort to an empty query if it does not. Similarly to how it's
    done in other places.
    
    Fixes: https://gitlab.gnome.org/GNOME/tracker/-/issues/215

 src/libtracker-data/tracker-sparql.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)
---
diff --git a/src/libtracker-data/tracker-sparql.c b/src/libtracker-data/tracker-sparql.c
index 1c04cc616..9b023a129 100644
--- a/src/libtracker-data/tracker-sparql.c
+++ b/src/libtracker-data/tracker-sparql.c
@@ -7029,9 +7029,19 @@ handle_property_function (TrackerSparql    *sparql,
                _append_string_printf (sparql, "FROM \"unionGraph_%s\" ",
                                       tracker_property_get_table_name (property));
        } else {
-               _append_string_printf (sparql, "FROM \"%s\".\"%s\" ",
-                                      tracker_token_get_idstring (&sparql->current_state.graph),
-                                      tracker_property_get_table_name (property));
+               const gchar *graph;
+
+               graph = tracker_token_get_idstring (&sparql->current_state.graph);
+
+               if (tracker_data_manager_find_graph (sparql->data_manager, graph)) {
+                       _append_string_printf (sparql, "FROM \"%s\".\"%s\" ",
+                                              graph,
+                                              tracker_property_get_table_name (property));
+               } else {
+                       /* Graph does not exist, ensure to come back empty */
+                       _append_string_printf (sparql, "FROM (SELECT 0 AS ID, NULL AS \"%s\" LIMIT 0) ",
+                                              tracker_property_get_name (property));
+               }
        }
 
        _append_string (sparql, "WHERE ID = ");